This invention is related to the field of ethylene polymer compositions. In particular, this invention is related to the field of crosslinked ethylene polymer compositions.
There has been, and continues to be, considerable interest in converting thermoplastic ethylene polymer compositions into thermosetting ethylene polymer compositions. This interest is due to the desire to combine the low cost, easy processing, and good mechanical properties of thermoplastic ethylene polymer compositions with the enhanced form stability at elevated temperatures, resistance to stress crack, and good tensile properties of thermosetting ethylene polymer compositions. Crosslinked ethylene polymer compositions are valuable because they can be used in the wire, cable, pipe, hose, and molded article industries.
Thermosetting ethylene polymer compositions are crosslinked by either chemical means or by radiation means. Currently, chemical crosslinking with organic peroxides is a preferred commercial approach. However, this approach has several drawbacks. One drawback is that organic peroxides can produce lower molecular weight radicals. These radicals can remove hydrogen atoms from an ethylene polymer and form low-boiling-point compounds. These low-boiling-point compounds can cause bubbles to form during the crosslinking reaction thereby adversely affecting the properties of the final ethylene polymer. This bubble drawback can be overcome to some extent by using a crosslinking co-agent that reduces bubble formation.
Crosslinking co-agents also have drawbacks. Currently, several commercial processes use aromatic-ring-containing, crosslinking co-agents. In particular, triallyl cyanurate (hereafter "TAC") has been used in several commercial processes. However, the use of TAC as a crosslinking co-agent has several drawbacks. One drawback is that the use of TAC is undesirable from a quality-control viewpoint. This is because the crosslinking efficiency is not consistent when using TAC. This means that a crosslinked product produced by using TAC can have widely varying physical properties. This is of particular concern in relation to the tensile properties of a crosslinked ethylene polymer composition.
Tensile properties are perhaps the most important indicators of the useful strength of a crosslinked ethylene polymer composition. For example, the tensile property known as "Percent Elongation at Break" is a good indicator of the brittleness of an ethylene polymer composition and the tensile property known as the "Strain Energy at Break" is a good indicator of the general toughness of an ethylene polymer composition. These properties are widely known and can be determined on commercially available test equipment. When TAC is used to produce a crosslinked ethylene polymer composition, these two tensile properties can vary widely thereby causing serious problems, in the area of quality control, for the producer.
This invention provides a solution to this problem of quality control.